The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A method and an apparatus of this kind are known from DE 197 12 829 A1 and are for example used for the determination of the position of the piston in pneumatic cylinders. For this purpose a magnetic element, for example a ring magnet or a disk magnet, is provided at the piston and produces a magnetic field which is detected by a magnetic sensor provided outside of the cylinder. In this arrangement the sensor is for example arranged at the outer wall of the cylinder in that its main axis of sensitivity lies parallel to the direction of movement of the piston. The magnetic field component detected by the sensor and lying parallel to the direction of relative movement of the piston and cylinder is a maximum when a magnet is directly moved passed the sensor. The determination of the maximum can be used to define a position recognition signal which can then serve as a switching point.
The magnetic field that is produced has a plurality of extremes, depending on the sensor element, which can lead to faulty switching when a secondary extremum is incorrectly interpreted as a main extremum. Only when a main extremum is present has the desired switching point however been reached. In order to overcome this problem the known solution proposes to use a second sensor which mainly responds to a magnetic field component lying perpendicular to the direction of relative movement. Its magnitude has a minimum value when the magnet directly passes the sensor. If the first sensor, which responds to the magnetic field component parallel to the direction of movement, just shows a maximum value then it can be determined by checking whether the second sensor just shows a minimum value or whether the maximum shown by the first sensor is a main extremum. In the known solution two different sensor signals accordingly have to be evaluated by two different sensors.
EP 0726448 B1 proposes to define a measurement window by additional sensors prior to and after the switching point in order to blend out possible auxiliary maxima. The measurement window is so arranged that only the main maximum lies in it. Other maxima, which are detected outside of the measurement window, are blended out and are not used as the switching point. Accordingly for this solution three different sensors and their signals have to be used.
Finally it is known from EP 0979988 B1 to use sensors which detect two field components standing perpendicular to one another. The quotient of the two signals is evaluated for the recognition of the relative position.
DE 10 2004 060249 A1 describes a sensor for another application with the aid of which the directional magnetic field is determined. The sensor has a planar soft magnetic structure with at least one first region and one second region which are separated from one another by a first long gap and a second long gap, with the first long gap and the second long gap not being parallel to one another. A first magneto-resistive element and a second magneto-resistive element are arranged along the first gap and the second gap and so designed that they can be electrically connected to a power source and to an electrical measurement unit. The soft magnetic structures serve as magnetic flux guides in order to concentrate the magnetic flux values which are to be detected and to enhance them. Fundamentally, only the component of an applied external magnetic field vector perpendicular to the longitudinal direction of a gap is notably enhanced by the magnetic field guides. Conclusions can be drawn regarding the magnetic field direction from the resistance signals of the magneto-resistive elements arranged in the gaps in DE 10 2004 060 249 A1.